Mineral Soil Fractions Research Articles

Spatial variability of soil mineral fractions and bulk density in Northern Ireland: Assessing the influence of topography using different interpolation methods and fractal analysis

Understanding how topography affects the distribution of soil properties is essential in the management of landscape hydrology and establishment of sustainable soil management practices. This study investigated the impact of topography on the variation in particle size distribution, coarse fragments, and soil bulk density using different interpolation techniques and fractal analysis. It also evaluated the performance of various interpolation techniques in predicting and characterizing the distribution of soil properties. The study was conducted using data from 620 samples extracted from the Tellus and LUCAS databases in Eglinton and Castlederg counties, Northern Ireland. Terrain attributes were obtained at a 30 × 30 m resolution using a global digital elevation model (GDEM) reintroduced to the Universal Transverse Mercator (UTM) projection. Interpolation analyses were conducted using inverse distance weighting (IDW), ordinary kriging (OK), block kriging (BK) and co-kriging (CK). Among the terrain attributes, elevation was the most influential covariate for CK. In addition, fractal analysis was conducted to assess the self-similarity of the soil properties. Prediction accuracy of the interpolation methods was evaluated using the Nash-Sutcliffe efficiency, mean absolute error, index of agreement, and Pearson correlation coefficient. Spatial maps produced from the kriging techniques showed high accuracy in the prediction of soil particle size distribution and bulk density. The use of elevation as an auxiliary variable was successful in producing accurate soil property distribution maps with CK. The fractal parameters showed that the soil properties had short range spatial variability, anti-persistent nature, and strong spatial structure. Additionally, the fractal dimension was strongly correlated with sand, silt and clay contents and bulk density, and weakly correlated with the coarse fragments.

The effects of tree-mycorrhizal type on soil organic matter properties from neighborhood to watershed scales

A large portion of terrestrial carbon (C) is stored as soil organic matter (SOM) so the balance between C inputs to SOM and soil C loss through respiration has important implications for global climate change. Tree-mycorrhizal association is a promising predictor of SOM dynamics, with the effect of ectomycorrhizal (ECM) trees and associated fungi scaling with the percent basal area of ECM-associated trees in a forest stand. However, uncertainty remains in the generality of ECM effects across the different spatial scales at which soil properties are known to vary. To determine the spatial variability of ECM effects on SOM pools within a forest stand, we used likelihood modelling techniques to compare non-spatial models, which average out ECM effects at the stand scale, against spatial models, which account for the size and spatial distribution of ECM-associated trees within a stand (i.e., the tree neighborhood). To investigate how watershed-scale variability in soil fertility and pH mediate ECM effects, we quantified forest floor leaf litter and soil properties in forest stands along mycorrhizal gradients within four adjacent watersheds that varied in parent material and rainfall. We found that ECM effects on SOM pools scaled with stand-level ECM dominance rather than varying based on individual trees at the neighborhood scale. Although forest floor leaf litter and mineral C:N ratios consistently increased with ECM dominance across watersheds, ECM effects on O horizon formation and the composition of SOM at depth differed among watersheds. Specifically, watersheds with lower soil pH and fertility exhibited greater changes along mycorrhizal gradients in O horizon depth, leaf litter δ15N and in the difference between δ15N of leaf litter and mineral soil fractions. Overall, this study shows the potential for intrinsic soil properties to mediate the effects of ECM trees and associated fungi on SOM formation and persistence in the tropics.

Modelling the sorption behaviour of perfluoroalkyl carboxylates and perfluoroalkane sulfonates in soils

A simple parametric model was developed to predict the sorption of perfluoroalkyl substances (PFASs) in soils. Initially, sorption and desorption solid-liquid distribution coefficients (Kd and Kd,des respectively) of eight PFASs (five perfluoroalkyl carboxylates, PFCAs, and three perfluoroalkane sulfonates, PFSAs) in seven soils with organic carbon (OC) content ranging from 1.6 to 41% were quantified using batch experiments. The information obtained helped to fill the gaps in a literature-based database of Kd values of PFASs, which was lacking data on soils with high OC content. The overall dataset finally comprised 435 entries. Normalized sorption coefficients for the soil OC and mineral fraction contents (KOC and KMIN respectively) were deduced for each PFAS by correlating the corresponding Kd values obtained under a wide range of experimental conditions with the fraction of organic carbon (fOC) of the soils. Furthermore, the sorption mechanisms in each phase were shown to depend mainly on PFAS chain length. The dependence of KOC and KMIN values on PFAS chain length defined the basic equations to construct the model for predicting PFAS sorption, applicable to both PFCAs and PFSAs with chain lengths ranging from 3 to 11 fluorinated carbons. The validation of the proposed model confirmed its ability to predict the Kd of PFASs based only on the soil OC and silt+clay contents and PFAS chain length. Therefore, it can be used in the first stages of a risk assessment process aiming at estimating the potential mobility of PFASs in soils after a contamination event. SynopsisThis study develops a new parametric model to predict the sorption of perfluoroalkyl substances (PFASs) in soils.

U- and Th- series disequilibria in separated soil mineral fractions: Insight into the mechanism and timescale of U, Th and Ra redistribution

In recent decades, U- and Th- series nuclides have been successfully used to determine weathering rates in soils and regoliths. However, the redistribution of the nuclides within secondary minerals and/or organic matter may interfere with the recorded U- and Th-series weathering signals and can result in inaccurate and/or poorly constrained weathering rate determinations. In addition, the migration of radionuclides in the environment represents a major current issue. The objective of this study, therefore, was to evaluate the meaning of bulk soil data for weathering investigations and to assess the processes and timescales of U, Th and Ra redistribution in a forested-soil profile, both at the bulk soil scale level and for separated mineral fractions. This study was carried out at the INRAE-Andra experimental beech-forested site of Montiers (Meuse, France), which was developed on acidic detrital sediments inherited from tropical paleosoils erosion. U- and Th-series activity nuclides were measured in bulk soil samples and in separated Fe nodules and clay-size (<2 μm) fractions. Furthermore, the analyses of atmospheric particles allow discussing the potential influence of atmospheric inputs.While bulk soil disequilibria suggested negligible to moderate mobility of the nuclides, an important transfer between mineral fractions was highlighted; this resulted in contrasting redistributions of U, Th and Ra within soil fractions. Fe-oxide nodules controlled the overall mobility of U on the bulk soil scale, while the fine <2 μm fraction controlled the mobility of Ra. A capture of 234U by Fe-oxide nodules (mainly comprised of goethite) was observed within the entire soil profile, most likely occurring for several hundred ky. In the shallow soil layers, a leaching of U from the <2 μm fraction that was not re-adsorbed onto Fe-oxides occurred. In contrast, Fe-oxides appeared to retain all U mobilized from the <2 μm fraction in the deep soil layers, resulting in negligible U mobility and an apparent “closed-system” at the bulk soil scale. This transfer, however, seemed to occur over shorter timescale than the one in the shallowest soil layers. No mobility of 226Ra was observed at the bulk soil scale, despite the large release of this nuclide from Fe-oxides. The measured (210Pb/226Ra) activity ratios suggested that up to 50% of 226Ra in Fe-oxides is available for desorption. However, the <2 μm fraction strongly readsorbed the released Ra and prevented the loss of this nuclide at the bulk soil scale. Considering the short half-life of 226Ra, such transfer from Fe-oxides to the <2 μm fraction occurred over a significantly shorter timescale than U mobilization. In addition, the analysis of the short-lived 228Ra isotope emphasized the flux involved in the biological cycle and the potential impact of vegetation on the soil radionuclide redistribution over a short timescale.Our results suggest that the fine <2 μm fractions might be suitable for probing the residence time of clay minerals in soils by the U-series dating method, if the soil profile can be assumed to consist of homogenous inherited clays.

Distribution of nickel (Ni) in peatland situated alongside mineral soil derived from ultrabasic rocks

&lt;p&gt;Detailed studies of Ni distribution in peat that is influenced by Ni-rich soil derived from ultrabasic rocks are still limited. The objective of this study was to reveal the characteristics of Ni in peat from Morowali (Central Sulawesi Province, Indonesia) at several depths and distances from the boundary of the ultrabasic mineral soil. Peat was sampled from depths of 0–30, 30–60, and 60–90 cm at distances of 100, 200, 300, 400, 500, and 600 m from the border of the ultrabasic mineral soil in March 2018. Ni characteristics were examined through their total, exchangeable, water-soluble, and adsorbed distributions. The relationships between Ni and some peat chemical properties such as pH; cation exchange capacity; macronutrient contents of K, Ca, and Mg; and micronutrient contents of Fe, Cu and Zn were also observed. The high Ni content in peat at the study transect is caused by an accumulation of Ni transported from elevated areas of mineral soil. Most Ni in peat is bonded to the soil organic exchange complexes. Accumulation of the mineral soil fraction in the peat surface is indicated at distances of 100–400 meters from the ultrabasic mineral soil. Ni distribution in peat at the study transect is mainly governed by a combination of Fe, pH, organic material, water content, peat depth, and distance from ultrabasic mineral soil.&lt;/p&gt;

Soil microbial communities influencing organic phosphorus mineralization in a coastal dune chronosequence in New Zealand.

The Haast chronosequence in New Zealand is an ∼6500-year dune formation series, characterized by rapid podzol development, phosphorus (P) depletion and a decline in aboveground biomass. We examined bacterial and fungal community composition within mineral soil fractions using amplicon-based high-throughput sequencing (Illumina MiSeq). We targeted bacterial non-specific acid (class A, phoN/phoC) and alkaline (phoD) phosphomonoesterase genes and quantified specific genes and transcripts using real-time PCR. Soil bacterial diversity was greatest after 4000 years of ecosystem development and associated with an increased richness of phylotypes and a significant decline in previously dominant taxa (Firmicutes and Proteobacteria). Soil fungal communities transitioned from predominantly Basidiomycota to Ascomycota along the chronosequence and were most diverse in 290- to 392-year-old soils, coinciding with maximum tree basal area and organic P accumulation. The Bacteria:Fungi ratio decreased amid a competitive and interconnected soil community as determined by network analysis. Overall, soil microbial communities were associated with soil changes and declining P throughout pedogenesis and ecosystem succession. We identified an increased dependence on organic P mineralization, as found by the profiled acid phosphatase genes, soil acid phosphatase activity and function inference from predicted metagenomes (PICRUSt2).

Effect of N and P fertilization on the allocation and fixation of photosynthesized carbon in paddy soil

ABSTRACT Photosynthetically fixed carbon (C) plays a crucial role in regulating soil organic carbon (SOC) sequestration in ecosystems. However, its allocation and fixation in SOC fractions in response to nitrogen (N) and phosphorus (P) application in paddy soil remains unclear. Thus, Potted rice seedlings independently treated with N, P, and NP were continuously 13CO2 labeled to investigated the influence of N and P application on the contribution of newly photosynthesized C to the rhizosphere versus bulk soil and particulate organic matter (POM) versus mineral fraction (MIN). N and NP enhanced net assimilated 13C on day14 (D14) (3.28 and 4.81 g13C m−2, respectively), with maximum C assimilation (16.64 g 13C m−2) occurring on day22 (D22) under NP. Aboveground biomass retained more 13C than belowground biomass for all treatments (55-77% vs 25-44 %). 13C incorporation into the rhizosphere exceeded that in bulk soil, with the maximum (6-10 %) found under N addition. The amount of newly assimilated 13C incorporated into POM increased in the rhizosphere under N and NP conditions, whereas MIN remained largely unaffected. 13C-MBC proportion in the total microbial biomass C (MBC) pool revealed that N and NP stimulated microbial activity to a greater degree than P. The main portion of 13C in the rhizosphere and bulk soil was found in POM on D14, which decreased over time due to microbial utilization. Contrastingly, root-derived 13C in the MIN remained unchanged between sampling days, which indicates that the stabilization of rhizodeposits in this fraction might be the potential mechanism underlying SOM sequestration in paddy soils.

Assessing changes in soil carbon stocks after land use conversion from forest land to agricultural land in Japan

In the context of their role in global climate change, changes in soil carbon stocks resulting from land use conversions from forest to agricultural lands (cropland and grassland) and subsequent agricultural activities were investigated with data from a total of 46 soil sampling sites across Japan (28 cropland and 18 grassland sites). Collection of soil sample pairs from forest land and adjacent agricultural land converted from forest land allowed derivation of the soil carbon change factor (SCCF; ratio of soil carbon stock in the agricultural land to that in the forest land at the time of soil sampling). Depth-based SCCFs for a fixed depth of 0–30 cm were 23% greater than mass-based SCCFs determined by the equivalent soil mass approach, in which soil carbon stocks in all replicated samples from forest land and agricultural land were calculated for different depths but equal masses of the soil mineral fraction. The overestimation of SCCFs by the depth-based approach was more pronounced in Andosols, particularly in croplands on Andosols, because soil bulk densities in forest lands were much lower than those in croplands on Andosols. The mass-based SCCFs for croplands (upland fields and orchards) and grasslands (pasture land and grazing land) after deforestation were 0.741 ± 0.259 (a 25.9 ± 25.9% decrease in soil carbon stocks after an elapsed time of 2–85 years) and 0.858 ± 0.212 (a 14.2 ± 21.2% decrease after an elapsed time of 26–81 years), respectively. Therefore, conversions from forest land to cropland or grassland resulted in a significant carbon loss from most soils in Japan. Soil carbon stocks in cropland converted from forest land declined as the elapsed time after deforestation increased although the uncertainty was large. Of the environmental variables, soil type (Andosols versus non-Andosols) and tillage frequency during cultivation had little impact on mass-based SCCFs; however, the SCCFs decreased with higher mean annual air temperature at the sites.

Adsorption-desorption behaviour of clomazone in Regosol and Chernozem agricultural soils

Studies of adsorption and desorption of pesticides by soils are important for understanding and predicting their fate and transmission in the environment. Considering the agricultural and environmental relevance of clomazone, its sorption?desorption behaviour was studied in two widespread Serbian agricultural soil types named Regosol and Chernozem. Both phenomena are well-described by the Freundlich equation, which shows that clomazone is generally sorbed more to organic matter than to the mineral soil fractions. Chernozem, a soil containing more of both organic matter and clay, was found to bind more, and desorb less herbicide, than Regosol. Higher desorption hysteresis obtained for Chernozem could be attributed to its larger number of high- -energy sorption sites, compared to Regosol. In both soils, the hysteresis effect increases with the rise of initial clomazone concentration in the soil-water system, while the percentage of desorbed amount during successive desorption cycles decreases. The presented adsorption?desorption study shows that soil composition plays an important role in clomazone behaviour and fate in the environment, and a significantly reduced probability of contamination of both the deeper soil layers and groundwater may be expected when this herbicide is found in humus-rich soils

Soil and Tree Nutrient Status of High Elevation Mixed Red Spruce (Picea rubens Sarg.) and Broadleaf Deciduous Forests

Anthropogenic and industrial emissions have resulted in historically high levels of acidic deposition into central Appalachian forests. Despite the reduction in acidic inputs due to legislation curbing industrial emissions in the United States, continued N deposition may impact forest ecosystems. Soil and foliar samples were collected from four high elevation red spruce sites along a modeled gradient of historic N deposition. The three most abundant tree species at all sites, Acer rubrum L., Betula alleghaniensis Britt., and Picea rubens Sarg., were sampled. Bulk soil beneath the canopies of individual trees were collected from the top 15-cm and separated into organic and mineral fractions for analysis. Mehlich-III soil extracts of soil fractions and foliar digests from these trees were subjected to elemental analysis. Soil N concentrations supported the presence of a N deposition gradient: in organic horizon soil fractions, N concentrations were driven by precipitation volume and elevation; whereas in mineral soil fractions, N concentration was explained by modeled N deposition rate and elevation. In organic fractions, significant reductions in Ca, K, and P were evident as N deposition increased, whereas the Ca:Sr ratio increased. Foliar Ca, K, and Sr declined in foliage with increasing N deposition, with concomitant increases in foliar Ca:Sr ratios. Although the three species were sympatric in mixed stands at all four sites, the foliar–soil nutrient associations differed among them across the gradient, indicating differential uptake and cycling of nutrients/metals by these forest tree species.

Predicting glyphosate sorption across New Zealand pastoral soils using basic soil properties or Vis–NIR spectroscopy

Glyphosate [N-(phosphonomethyl) glycine] is the active ingredient in Roundup, which is the most used herbicide around the world. It is a non-selective herbicide with carboxyl, amino, and phosphonate functional groups, and it has a strong affinity to the soil mineral fraction. Sorption plays a major role for the fate and transport of glyphosate in the environment. The sorption coefficient (Kd) of glyphosate, and hence its mobility, varies greatly among different soil types. Determining Kd is laborious and requires the use of wet chemistry. In this study, we aimed to estimate Kd using basic soil properties, and visible near-infrared spectroscopy (vis–NIRS). The latter method is fast, requires no chemicals, and several soil properties can be estimated from the same spectrum. The data set included 68 topsoil samples collected across the South Island of New Zealand, with clay and organic carbon (OC) contents ranging from 0.001 to 0.520 kg kg−1 and 0.021 to 0.217 kg kg−1, respectively. The Kd was determined with batch equilibration sorption experiments and ranged from 13 to 3810 L kg−1. The visible near-infrared spectra were obtained from 400 to 2500 nm. Multiple linear regression was used to correlate Kd to oxalate extractable aluminium and phosphorous and pH, which resulted in an R2 of 0.89 and an RMSE of 259.59 L kg−1. Further, interval partial least squares regression with ten-fold cross-validation was used to predict Kd by vis–NIRS, and an R2 of 0.93 and an RMSECV of 207.58 L kg−1 were obtained. Thus, these results show that both basic soil properties and vis–NIRS can predict the variation in Kd across these samples with high accuracy and hence, that glyphosate sorption to a soil can be determined with vis–NIRS.

Contrast-enhanced repacked soil cores as a proxy for soil organic matter spatial arrangement

Soil organic matter (SOM) plays a key role in soil structure formation, although the bidirectional relationship between SOM and the soil pore network is complex and needs further investigation. Despite great advances provided by X-ray computed microtomography (µCT), it has only been used in a few studies to investigate the organic matter 3D-arrangement within the soil matrix. Results are based on the X-ray linear attenuation coefficient (α), and mixtures of organic and mineral soil fractions could imply overlapping of information that makes any segmentation procedure difficult. The aim of this study was to visualise, segment, and quantify the particulate organic matter fraction (POM) within the soil matrix through X-ray µCT in combination with contrast agents (phosphomolybdic acid and silver nitrate). Two series of repacked soil cores, ‘dry’ and ‘wet’, were scanned through X-ray µCT at a 7-µm resolution. Different segmentation approaches were tested to separate POM from other soil phases: manual, global, and local thresholding methods. Reported algorithms were also compared with a supervised grey value-based (GV) approach followed by morphological operations. Results showed contrast agents increased α of POM, simplifying its identification and the following segmentation on dry cores. The POM was discriminated from the mineral fraction and its content correctly estimated. This was particularly accurate when applying manual thresholding or GV approach with respect to indicator kriging, suggesting that operator-based ability to set threshold level is still the best solution for accurate POM segmentation. Beyond single-phase accounting, different thresholding algorithms and morphological operations also affected POM morphological characteristics. In particular, the simpler was an object shape, the easier was its segmentation. Improvements are thus required to increase the efficiency of automated thresholding algorithms. Moreover, wet cores were exposed to washing-out phenomena that compromised any digital image processing and further POM quantification, implying that more effort should be made to find other suitable staining agents.

Consistent proteinaceous organic matter partitioning into mineral and organic soil fractions during pedogenesis in diverse ecosystems

Proteinaceous compounds are critical in soil organic matter (SOM) formation and persistence, but the partitioning into mineral-associated and organic forms during hundreds and thousands of years of pedogenesis are poorly understood across multiple climates and vegetation types. We investigated the partitioning of amino acids (AA) into mineral-bound (MB) and non-mineral associated organic (NMO) soil fractions to discern whether consistent patterns during ecosystem development were observed across two different climates (cool temperate continental, USA; and moist oceanic forests, New Zealand). Although each ecosystem retained unique soil AA signatures, consistent patterns in both systems were observed with three main findings. (1) Regardless of differences in climate and vegetation between the two ecosystems, AA consistently partitioned in similar ways into MB and NMO soil fractions. For example, Thr, Ser, and Asx were relatively more dominant in the NMO fraction while Arg, Lys, Cys, and Met were relatively more dominant in the MB soil fraction. (2) AA change, showed similar trends related to chemical groupings of positively-charged, polar aromatic, sulfur containing, and non-polar AAs across both ecosystems consistent with changing patterns of soil Fe and Al bearing minerals, such as an increasing weathering index (WI; Fe dithionite/total Fe) and losses of Fe and Al from surface soils during ecosystem development. (3) The pedogenic patterns of AA change, in each system paralleled biological transitions in bacterial communities, suggesting a linkage between the AA sources and the soil sink that contributes to soil organic N. This latter point contrasts with the potential for complete change in soil AA composition that could occur upon processing and binding in soil. Overall, the consistency in the types of AAs that partition into either MB or NMO soil fractions across locations provide evidence of similar processes that contribute to soil organic N accrual across soils. Some AA also appeared to be retained in soil by electrostatic forces, and AA–organic matter interactions, that need further study. Mineral–metal complexation of AAs with sulfur side chain groups, and non-polar interactions, respectively, provide examples of these potential mechanisms for study. The results not only support mechanisms of soil proteinacous organic matter chemistry being driven by the interactions with the soil matrix, acting as a “sink”, but also draw attention to the sources of organic matter (microbes, plants) in determining the composition of organic N in soil during pedogenesis.

Microbial community assembly differs across minerals in a rhizosphere microcosm.

Mineral-associated microbes drive many critical soil processes, including mineral weathering, soil aggregation and cycling of mineral-sorbed organic matter. To investigate the interactions between soil minerals and microbes in the rhizosphere, we incubated three types of minerals (ferrihydrite, kaolinite and quartz) and a native soil mineral fraction near roots of a common Californian annual grass, Avena barbata, growing in its resident soil. We followed microbial colonization of these minerals for up to 2.5 months - the plant's lifespan. Bacteria and fungi that colonized mineral surfaces during this experiment differed across mineral types and differed from those in the background soil, implying that microbial colonization was the result of processes in addition to passive movement with water to mineral surfaces. Null model analysis revealed that dispersal limitation was a dominant factor structuring mineral-associated microbial communities for all mineral types. Once bacteria arrived at a mineral surface, capacity for rapid growth appeared important, as ribosomal copy number was significantly correlated with relative enrichment on minerals. Glomeromycota (a phylum associated with arbuscular mycorrhizal fungi) appeared to preferentially associate with ferrihydrite surfaces. The mechanisms enabling the colonization of soil minerals may be foundational in shaping the overall soil microbiome composition and development of persistent organic matter in soils.

Abundance of lipids in differently sized aggregates depends on their chemical composition

Evidence for a vital role of soil mineral matrix interactions in lipid preservation is steadily increasing. However, it remains unclear whether solvent-extractable (‘free’) or hydrolyzable (‘bound’) lipids, including molecular proxies, e.g., for cutin and suberin, are similarly affected by different stabilization mechanisms in soil (i.e., aggregation or organo-mineral association). To provide insights into the effect of these stabilization mechanisms on lipid composition and preservation, we investigated free and bound lipids in particulate and mineral soil fractions, deriving from sand- and silt-/clay-sized aggregates from a forest subsoil. While free lipids accumulated in sand-sized aggregates, the more complex bound lipids accumulated in silt- and clay-sized aggregates, particularly in the respective mineral fractions < 6.3 µm (fine silt and clay). The presence of both, cutin and suberin markers indicated input of leaf- and root-derived organic matter to the subsoil. Yet, our cutin marker (9,10,ω-trihydroxyoctadecanoic acid) was not extracted from the mineral aggregate compartments < 6.3 µm, perhaps due to its chemical structure (i.e., cross-linking via several hydroxy groups, and thus higher ‘stability’, in macromolecular structures). Combined, these results suggest that the chemical composition of lipids (and likely also that of other soil organic matter compounds) governs interaction with their environment, such as accumulation in aggregates or association with mineral soil compartments, and thus indirectly influences their persistence in soil.

Promoting revegetation and soil carbon sequestration on decommissioned forest roads in Colorado, USA: A comparative assessment of organic soil amendments

Forest roads are commonly decommissioned and revegetated to decrease erosion, prevent weed encroachment, manage recreation and improve overall watershed condition on federal lands, but may also provide a complementary opportunity to sequester carbon (C) in soils. Soils on decommissioned roads are typically compacted with limited capacity for water retention, decreased mineral nitrogen (N) availability and low organic matter content, impairing revegetation and soil C sequestration efforts. We evaluated the effects of an organic fertilizer, wood strand mulch and a woody biochar on soil physical, chemical and biological processes to improve revegetation and C sequestration on decompacted forest roads. We monitored plant and soil responses to the treatments and their combinations over three growing seasons on four decommissioned road segments in northern Colorado. The organic fertilizer increased plant available mineral N for the first year of the study and resulted in a 21% increase in total plant cover and 67% increase in root biomass. The wood strand mulch increased total plant cover and root biomass to a similar extent, but had no effect on soil water content or mineral N availability. Instead, mulch stimulated soil microbial respiration and increased soil C content, two of the best predictors of plant cover and biomass. The woody biochar increased soil water content by 26% and elevated mineral N availability throughout the study, but did not improve plant cover, above- or belowground biomass. Mulch, biochar and their combined treatments sequestered C, but through distinct pathways. Microbial processing of wood strand mulch added C to the mineral soil fraction, whereas biochar added C directly to the coarse particulate fraction with no effect on mineral soil C or soil respiration. Restoration practitioners can utilize these results to inform management decisions and guide further research on different rates and combinations of organic amendments to revegetate and sequester C on decommissioned forest roads.

Centennial Fertilization-Induced Soil Processes Control Trace Metal Dynamics. Lessons from a Long-Term Bare Fallow Experiment

Long-term bare fallow (LTBF) experiments with historical sample archives offer unique opportunities to study long-term impacts of anthropogenic activities on mineral soil fractions. In natural agro- and ecosystems, such impacts are often masked by organic matter due to its buffering action and rapid turnover. The 42-plot LTBF trial of INRA (Institut National de la Recherche Agronomique) started in Versailles (France) in 1928 to assess the impacts of prolonged application of fertilizers and amendments on the composition and properties of loamy soils. Here, we established geochemical budgets of major and trace elements on surface samples from 1929 and 2014 for four groups of treatments relevant for developed soil processes. We considered accompanying effects of soil compaction or decompaction due to changing physicochemical conditions over 85 years. Element losses from the surface horizon were quantified via fertilization-induced or -amplified soil processes: clay leaching favored by Na- or K-based fertilization, and lixiviation of major and trace elements in acidic or alkaline soil conditions. Enhanced mineral weathering was shown for acidified and nonamended plots. Conclusions on trace metal migration were confirmed by selected analyses on subsurface horizons. Additional information was provided on specific element inputs via fertilizers and/or diffuse inputs via atmospheric deposition.

Plant litter chemistry alters the content and composition of organic carbon associated with soil mineral and aggregate fractions in invaded ecosystems.

Through the input of disproportionate quantities of chemically distinct litter, invasive plants may potentially influence the fate of organic matter associated with soil mineral and aggregate fractions in some of the ecosystems they invade. Although context dependent, these native ecosystems subjected to prolonged invasion by exotic plants may be instrumental in distinguishing the role of plant-microbe-mineral interactions from the broader edaphic and climatic influences on the formation of soil organic matter (SOM). We hypothesized that the soils subjected to prolonged invasion by an exotic plant that input recalcitrant litter (Japanese knotweed, Polygonum cuspidatum) would have a greater proportion of plant-derived carbon (C) in the aggregate fractions, as compared with that in adjacent soil inhabited by native vegetation that input labile litter, whereas the soils under an invader that input labile litter (kudzu, Pueraria lobata) would have a greater proportion of microbial-derived C in the silt-clay fraction, as compared with that in adjacent soils that receive recalcitrant litter. At the knotweed site, the higher C content in soils under P.cuspidatum, compared with noninvaded soils inhabited by grasses and forbs, was limited to the macroaggregate fraction, which was abundant in plant biomarkers. The noninvaded soils at this site had a higher abundance of lignins in mineral and microaggregate fractions and suberin in the macroaggregate fraction, partly because of the greater root density of the native species, which might have had an overriding influence on the chemistry of the above-ground litter input. At the kudzu site, soils under P.lobata had lower C content across all size fractions at a 0-5cm soil depth despite receiving similar amounts of Pinus litter. Contrary to our prediction, the noninvaded soils receiving recalcitrant Pinus litter had a similar abundance of plant biomarkers across both mineral and aggregate fractions, potentially because of the higher surface area of soil minerals at this site. The plant biomarkers were lower in the aggregate fractions of the P.lobata-invaded soils, compared with noninvaded pine stands, potentially suggesting a microbial co-metabolism of pine-derived compounds. These results highlight the complex interactions among litter chemistry, soil biota, and minerals in mediating soil C storage in unmanaged ecosystems; these interactions are particularly important under global changes that may alter plant species composition and hence the quantity and chemistry of litter inputs in terrestrial ecosystems.

Why does carbon increase in highly weathered soil under no-till upon lime and gypsum use?

Field experiments have been used to explain how soil organic carbon (SOC) dynamics is affected by lime and gypsum applications, however, how SOC storage occurs is still debatable. We hypothesized that although many studies conclude that Ca-based soil amendments such as lime and gypsum may lead to SOC depletion due to the enhancement of microbial activity, the same does not occur under conservation agriculture conditions. Thus, the objective of this study was to elucidate the effects of lime and gypsum applications on soil microbial activity and SOC stocks in a no-till field and in a laboratory incubation study simulating no-till conditions. The field experiment was established in 1998 in a clayey Oxisol in southern Brazil following a completely randomized blocks design with a split-plot arrangement and three replications. Lime and gypsum were surface applied in 1998 and reapplied in 2013. Undisturbed soil samples were collected before the treatments reapplications, and one year after. The incubation experiment was carried out during 16months using these samples adding crop residues on the soil surface to simulate no-till field conditions. Lime and gypsum applications significantly increased the labile SOC stocks, microbial activity and soil fertility attributes in both field and laboratory experiments. Although the microbial activity was increased, no depletion of SOC stocks was observed in both experiments. Positive correlations were observed between microbial activity increase and SOC gains. Labile SOC and Ca2+ content increase leads to forming complex with mineral soil fractions. Gypsum applications performed a higher influence on labile SOC pools in the field than in the laboratory experiment, which may be related to the presence of active root system in the soil profile. We conclude that incubation experiments using lime and gypsum in undisturbed samples confirm that soil microbial activity increase does not deplete SOC stocks under conservation agriculture.

Factors influencing the molecular composition of soil organic matter in New Zealand grasslands

Few studies have attempted to simultaneously link multiple environmental factors to the composition of soil organic matter (SOM). This knowledge gap challenges the ability to predict the response of SOM to environmental changes and make recommendations for effective land-based C sequestration strategies. In this study, topsoil samples from the New Zealand National Soils Archive, of different soil orders and obtained from different climatic regions, land-uses, and positions in the landscape, and taken at two sampling times (one from 1960s to early 1990s and the other after 2004) were analysed for SOM composition using Pyrolysis-GC/MS after being treated with HF to minimise the interference of the soil mineral fraction. Principal component analysis and redundancy analysis were used to screen out major environmental factors that impact on SOM composition. Ordination of SOM composition generally followed two gradients (canonical axes):decomposition and fresh input, and was significantly explained by the C to N ratio and total C (Monte Carlo permutation test, P<0.01) with other factors, such as average annual temperature, average annual rainfall rate, total N and HF-removable C being somehow redundant with total C. Soil order, landscape type and land-use type further contributed to the explanation of the variability of SOM composition, although they were largely redundant with the above-mentioned factors. Overall, sampling time showed little effect on SOM composition. Given the large variation of SOM composition for different soils (e.g. Allophanic soils having a larger fraction of decomposed organic matter compared to other soil orders) and different mechanisms governing SOM stabilisation, effective C sequestration strategies should be case-specific for New Zealand grasslands.